Metal and composite golf club shaft

ABSTRACT

A golf club shaft is provided including a metal tip section and a composite butt section. The butt section includes a reduced diameter portion telescopically received within an axial bore of the tip section. An adhesive is disposed between the tip section and the butt section to secure the two together. An insulating layer may be disposed between the tip section and the butt section to prevent galvanic corrosion.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a golf club shaft having improvedperformance characteristics and more particularly to a two piece golfclub shaft combining a metal portion and a composite portion whichretains the advantages of both materials while eliminating theirdisadvantages.

[0002] Control and accuracy in the game of golf is influenced by thetorsional stiffness of the shaft. The torsional stiffness of the shaftresists twisting of the club head during the swing and particularly whenthere is less than perfect contact between the golf ball and club head.Metal golf club shafts, the most popular metal being steel, are used bymany golfers. An advantage of steel shafts is their high torsionalstiffness which are known in the relevant art as “low torque” shafts.

[0003] Many golf shaft manufacturers offer composite shafts, oftenreferred to as graphite shafts, which are usually made from a compositeof graphite or carbon fiber and epoxy resin. Composite shafts can besignificantly lighter than metal shafts, but the torsional stiffness ofa conventional graphite shaft is less than that of a steel shaft.Graphite shafts are therefore known in the art as ‘higher torque’ thansteel shafts.

[0004] In an attempt to torsionally stiffen composite shafts,manufacturers have used different fiber types such as high moduluscarbon, aramid and boron fibers. They have also varied the constructionby wrapping the fibers at different angles in an attempt to improvetorsional stiffness. Most of these changes have increased the cost andoften had a negative effect on the playability of the shaft.

[0005] Recent studies have shown that only the tip section of the shaftprovides the torsional resistance to prevent club head twisting due topoor ball/head contact. Contact between the club head and ball is a verybrief dynamic event and only the tip section of the shaft gets loadedduring this time period. The event is effectively over before the fulllength of the shaft is loaded. It is therefore desirable to constructthe tip section of the shaft from metal which has a high torsionalstiffness, such as steel, whereas the butt section can be constructedfrom a composite, such as graphite, with a lower torsional stiffness. Insuch a manner, the effective torque characteristic of the shaft can beenhanced while maintaining a light weight.

[0006] One attempt to combine the advantages of metal and compositeshafts is disclosed in U.S. Pat. No. 4,836,545 to Pompa. However, Pompadoes not describe the affect the physical characteristics of the twoshaft sections has on shaft performance. The length and weight of thetwo sections of the Pompa shaft were arbitrarily selected. Testing alsorevealed that the weight of the metal tip section was extremely heavy ascompared to the weight of the composite butt section. By having a tipsection that was too heavy, the center of gravity (CG) of the shaftmoved towards the tip end and undesirably increased the swing weight ofthe club.

[0007] Swing weight is a measure of the static moment of the assembledclub about a point usually 14″ from the grip end. The absolute weightand balance point (CG position) of head, shaft and grip all affect clubswing weight. Although it is common for major club manufacturers tospecify head weights to achieve desired club swing weights knowing thespecifications of the shaft and grip, this approach is not alwayspossible. Component suppliers typically offer club head weights whichconform to industry accepted weight ranges. As such, it is highlydesirable to provide shafts that may be used with such heads to achievepopular club swing weights.

[0008] Further, it is undesirable for the head to receive secondaryweighting to achieve a desired club swing weight. Secondary weighting isusually introduced at the extreme tip end of the shaft where the shaftis inserted in the hosel of the head. This positioning is not optimalbeing away from the head center of gravity and thus reduces momentumtransfer to the ball for a given swing speed. Less momentum transfer tothe ball reduces the distance the ball will travel.

[0009] Pompa also failed to appreciate the difficulty of joining themetal and composite sections of the shaft. The joint must becosmetically acceptable and strong enough to prevent failure.

[0010] In view of the foregoing, it would be desirable to provide animproved golf club shaft that exhibits the advantages of both metal andcomposite shafts while eliminating their respective disadvantages.

SUMMARY OF THE INVENTION

[0011] It is a primary purpose and principal objective of the presentinvention to provide a golf club shaft combining the separate advantagesof metal and composite shafts into a single, hybrid design.

[0012] It is another objective of the present invention to eliminate theseparate disadvantages of metal and composite shafts in a single, hybriddesign.

[0013] It is yet another objective of the present invention to provide amethod for manufacturing a golf club shaft having a tip section of ametallic material and a butt section of a composite material.

[0014] It is still yet another objective of the present invention toprovide a method and device for achieving the above objectives whileconforming to the rules of golf as defined by the United States GolfAssociation.

[0015] According to one embodiment of the present invention, a golf clubshaft is provided including a metal tip section and a composite buffsection. The cylindrical tip section is formed of a metallic materialsuch as steel. The cylindrical butt section is formed of a compositematerial such as graphite and includes a reduced diameter portion orplug formed at an end thereof. The plug of the butt section istelescopically received in the end of the tip section such that the endof the tip section overlaps the reduced diameter portion of the buttsection. An adhesive, such as epoxy, is disposed between the tip andbutt sections to secure the two sections together.

[0016] In another embodiment of the present invention, an insulatinglayer is disposed between the tip and butt sections to prevent metal tocomposite contact within the metal/composite joint. By preventing metalto composite contact, the insulating layer reduces or eliminates thepotential for galvanic corrosion within the joint. The insulating layermay be formed as a plurality of insulating spacers such as glass beadsor a glass layer built into the butt section within the joint.

[0017] The invention will be better understood and further objects andadvantages thereof will become more apparent from the ensuing detaileddescription taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic side elevation view of a golf club shaftincorporating the teachings of the present invention;

[0019]FIG. 2 is a graph showing the relationship of torsional resistanceto metal tip length;

[0020]FIG. 3 is a graph showing the relationship of torsional resistanceto metal and composite tip lengths;

[0021]FIG. 4 is a graph showing the relationship of shaft weight to tiplength and its affect on club swing weight;

[0022]FIG. 5 is a detailed schematic cross sectional view of ametal/composite joint of the golf club shaft of FIG. 1;

[0023]FIG. 6 is a graph showing the relationship of the tip bendingstiffness of the shaft of the present invention to that of aconventional steel shaft;

[0024]FIG. 7 is a graph showing the relationship of the tip wallthickness of the shaft of the present invention to that of aconventional steel shaft;

[0025]FIG. 8 is a graph showing the relationship of the butt wallthickness of the shaft of the present invention to that of aconventional graphite shaft;

[0026]FIG. 9 is a detailed schematic cross sectional view of ametal/composite joint of the golf club shaft of FIG. 1 according to asecond embodiment of the present invention;

[0027]FIG. 10 is a detailed schematic cross sectional view of ametal/composite joint of the golf club shaft of FIG. 1 according to athird embodiment of the present invention; and

[0028]FIGS. 11a and 11 b are detailed schematic cross sectional views ofa metal/composite joint of the golf club shaft of FIG. 1 according to afourth embodiment of the present invention.

DETAILED DESCRIPTION

[0029] Referring to FIG. 1, there is shown a golf club 10 having a grip12, a head 14 and a tubular shaft 16. Although the club 10 isillustrated as a wood, it may also be an iron or a putter. The shaft 16includes a tip section 18 and a butt section 20. The tip section 18 ispreferably formed of a metallic material such as high strength steelwhile the butt section 20 is preferably formed of a composite materialsuch as graphite. While the shaft 16 has been illustrated as having asmooth, tapered sidewall 22, it should be appreciated that a parallel orstepped sidewall could substitute therefore.

[0030] The tip section 18 is secured at a lower end 24 to head 14 bysizing it to fit standard club head hosel sockets. The upper end 26 oftip section 18 is telescopically and slidingly fit over the lower end 28of the butt section 20. The physical characteristics of the tip section18 from head 14 to the joint 30 where it meets the butt section 20, aredesigned to provide desired balance of torsional stiffness, bendingstiffness (flex), strength, and weight in order to yield the bestplayability when combined with the composite butt section.

[0031] The relationship between the physical characteristics of the tip18 and the playability of the shaft 16 is complex and many factors mustbe taken into consideration.

[0032] 1) As the metallic tip section 18 is shortened, the torsionalstiffness it provides becomes less significant. As the tip section 18 islengthened, the weight of the tip section 18 becomes more significant.

[0033] 2) It is desirable to retain an industry standard diameter ofeither 0. 335 or 0.350 inches at the lower end 24 of the tip section 18to allow fitment of industry standard club heads. However, the diametercan be increased towards the upper end 26 of the tip section 18 toincrease both torsional and bending stiffness.

[0034] 3) For the same weight tip section 18, increasing the diameter atthe upper end 26 decreases the wall thickness and reduces durability.

[0035] 4) To minimize the weight of the tip section 18, the wall of thetip section 18 can be made thinner. However, as the wall thickness isreduced, the strength and stiffness of the tip section 18 is reduced.

[0036] 5) As the diameter and wall thickness of tip section 18 isvaried, the bending stiffness (flex) is also changed. If the bendingstiffness (flex) is too high or too low the playability and feel of theshaft becomes unacceptable.

[0037] Extensive playability and durability testing has allowed anacceptable geometry range and a preferred geometry to be defined for thetip section 18 of a 46 inch wood shaft weighing between 65 g and 90 g.

[0038] If the length of tip section 18 is less than 6 inches it does notprovide sufficient torsional stiffness to improve shot accuracy. FIG. 2shows how the torque of the club, as measured using a torque test, isreduced as the length of the tip section is increased. FIG. 3 shows thelower torque characteristics of a steel tip compared to a graphite tip.

[0039] If the tip section 18 is greater than 12 inches, the shaft weightis undesirably increased and the center of gravity position of the shaft16 is moved too far towards the tip end 24. FIG. 4 shows how the shaftweight increases with tip length. FIG. 4 also shows that with shaftweights for tip lengths between 6 and 12 inches it is possible toachieve club swing weights ranging from D1 to D5 using an industryaccepted head weight.

[0040] If the diameter of the upper end 26 of the tip section 18 isincreased above 0.415 inches, the bending stiffness becomes undesirablyhigh adversely affecting tip flexibility and providing a low balltrajectory and a harsh feel to the club. Likewise, if the diameter ofthe upper end 26 of the tip section 18 is reduced below 0.385inches, thebending stiffness is undesirably low providing a high ball trajectoryand too soft a feel to the club.

[0041] Durability testing carried out with an air cannon has shown thatdiameters above 0.415 inches for the upper end 26 of tip section 18 witha length range from 6 to 12 inches and an overall weight of shaft 16 ofless than 65 g does not provide a tip section 18 with sufficientdurability.

[0042] In the preferred embodiment, a 46 inch wood shaft weighs 75 g andhas a tip section 18 with a length of 8 inches and a diameter at theupper end 26 of .4 inches and a diameter of 0.335 or 0.350 inches at thelower end 24. Such a preferred tip section 18 has a torque of less than0.6 degrees over the 8 inch length when measured using a torque test.This compares with a torque of greater than 1.5 degrees for the tipsection of a typical graphite shaft measured using the same test method.

[0043] Turning now to FIG. 5, the joint 30 of FIG. I is illustrated ingreater detail. An important aspect affecting the durability of theshaft 16 is the strength of the joint 30 between the metal tip section18 and the composite butt section 20. As can be seen, the tip section 18is in the form of a hollow metal cylinder and the butt section 20 isformed as a hollow composite cylinder. The butt section 20 includes areduced diameter cylindrical portion or plug 32 for insertion into thetip section 18. The reduced diameter portion 32 may be formed during thelay-up of the composite butt section 20 or may be formed by grindingaway a pre-selected annular amount of the butt material after initialformation. The reduced diameter portion 32 is dimensioned to ensure asufficient overlap and durable interconnection with the tip section 18.

[0044] The metal tip section 18 and composite butt section 20 are joinedtogether with an adhesive, such as epoxy bond 31. The thickness of theadhesive 31 is carefully controlled and the surface area of the tipsection 18 and butt section 20 along the adhesive 31 is sufficient toensure adequate strength. Bond strength is selected such that the joint30 does not fail in shear from the torsional loads imposed throughgenerally accepted levels of abuse while playing the game of golf.Limiting the maximum thickness of the adhesive 31 and increasing thesurface area of the joint 30 also maintains the highest straightnessstandard for the assembled shaft 16.

[0045] Static and dynamic durability testing has shown that bondthickness should be controlled to between 0.003″ and 0.006″. Testing hasalso shown that for a metal tip section 18 with a diameter at the upperend 26 of between 0.385″ and 0.415″ the composite butt section 20 shouldbe inserted into the metal tip section 18 between about 0.75″ and about1.5″ to provide an adequate bond area. In the preferred embodiment, a 46inch shaft driver has a bond thickness of .0045″ and the composite buttsection 20 is inserted 1.25″ into the metal tip section 18. Suchgeometry has been proven to provide adequate strength and straightnessin the assembled shaft 16.

[0046] The overall bending stiffness of the shaft 16, which defines theshaft flex, is influenced by the design of the tip section 18, the buttsection 20 and the geometry of the joint 30. Local stiffness in thejoint could be high and the length of the joint 30 must be such toprovide sufficient durability while not being excessively stiff.

[0047] Flex ranges for various categories of players with differentswing characteristics are generally accepted throughout the industrywith those provided by True Temper Dynamic (trademark) shafts oftenbeing used as a point of reference. Using the geometry range and overallshaft weights defined above, FIG. 6 shows the bending stiffness of shaft16 through the joint compares favorably to that of a Dynamic shaftensuring excellent feel and desirable ball flight. The stiffness of theshaft in FIG. 6 is measured as the tip deflection in a simple cantileverload test.

[0048]FIGS. 7 and 8 compare the wall thickness along the length of themetal tip section 18 and composite butt section 20 in the preferredembodiment of shaft 16 with the wall thickness found in a popular TrueTemper Dynamic (trademark) steel shaft and a popular Grafalloy Prolite(trademark) graphite shaft. It will be apparent that the wall thicknessin the shaft 16 is very different to that in the available True Tempersteel and Grafalloy graphite shafts. This illustrates that the shaft 16cannot be made by bonding together tip and butt sections cut fromcommercially available steel and graphite shafts.

[0049] Referring again to FIG. 5, the formation of the reduced diameterportion 32 also defines an edge in the form of a radial wall 34 in thebutt section 20. Although the radial wall 34 is illustrated as extendingorthogonally to the reduced diameter portion 32, the radial wall 34 mayalso be formed at an acute or obtuse angle relative thereto. The radialwall 34 is preferably dimensioned so as to be equal to or slightlygreater than the sum of the thickness of the end 38 of the tip section18 and the thickness of the adhesive 31 so as to yield a smooth-wall,concentric transition between the tip section 18 and the butt section 20along the perimeter of the shaft 16 adjacent the joint 30.

[0050] Turning now to FIG. 9, a second embodiment of the presentinvention is illustrated. In this embodiment, the components which arethe same as those in the previous embodiment are identified with thesame reference numeral but increased by 200. The second embodimentdiffers from the previous embodiment by the insertion of an insulatinglayer 252 in the form of a plurality of spacers between the tip section218 and the butt section 220. The insulating spacers 252 are preferablyin the form of beads and are preferably formed of an insulating materialsuch as ceramic or glass. The insulating beads 252 prevent the metal ofthe tip section 218 from contacting the graphite of the butt section 220to reduce or eliminate galvanic corrosion within the joint 230.

[0051] The beads 252 also help control the alignment and separation ofthe tip section 218 relative to the butt section 220. In this regard,the diameter of the beads 252 is selected in accordance with the gap 242so as to provide sufficient space for the adhesive 231 between the beadswhile also coaxially aligning the tip section 218 with the butt section220 so as to ensure a smooth perimeter surface along the shaft 216adjacent the joint 230. Preferably, the beads 252 are pre-mixed with theadhesive 246 prior to its application within the joint 230.

[0052] Turning now to FIG. 10, a third embodiment of the presentinvention is illustrated. In this embodiment, the components which arethe same as those in the previous embodiments are identified with thesame reference numeral but increased by 300. The third embodimentdiffers from the previous embodiments by the inclusion of an insulatinglayer 352 in the form of an overlayer between the tip section 318 andthe butt section 320. The overlayer 352 is preferably in the form of aninsulating layer integrally formed along the outboard surface of thereduced diameter portion 332 and is preferably formed of an insulatingmaterial such as ceramic or glass to prevent the metal of the tipsection 318 from contacting the graphite of the butt section 320 toreduce or eliminate galvanic corrosion within the joint 330. The layer352 also helps control the alignment and separation of the tip section418 relative to the butt section 320. The layer 352 is preferably formedby forming the butt section 320 with a relatively thick layer of glassat one end thereof prior to the formation of the reduced diameterportion 332. The reduced diameter portion 332 is then formed by grindingaway a pre-selected annular amount of the glass so as to leave the layer352 as an outboard surface of the reduced diameter portion 332. In thisway, the fabric 352 isolates the remaining composite material of thebutt section 320 from the metallic material of the tip section 318.

[0053] Referring to FIGS. 11 a and 11 b, in production shafts, a plasticferrule 500 is incorporated in the joint 430 between the steel tip 418and the graphite butt section 420. The ferrule 500, is made fromsuitable extruded or injection molded plastic and is used to accommodateany slight geometrical misalignment between the graphite section 420 andsteel section 418. The outside diameter of the ferrule 500 is sized tobe slightly larger than the diameter of either the graphite section 410or steel section 418. After assembly, excess material can be removedfrom the plastic ferrule 500 either by buffing on a fine abrasive beltor by wiping with a solvent such as acetone. Removing material from theferrule 500 in this way can provide a smooth transition on the outsidesurface between the steel and graphite sections 418 and 420.

[0054] The cross section of the ferrule 500 can be altered from therectangular form in FIG. 11 a to incorporate inwardly sloping faces asshown in FIG. 11 b. Such ferrule geometry can be used to accommodate asmall radius in the corner of the machined graphite butt section 420that can ease the manufacture of the shaft 416.

[0055] Referring again to FIGS. 1 and 2, the steps for manufacturing theshaft 16 will be described. The hollow cylindrical butt section 20 isformed to a given length by arranging a plurality of layers of apre-selected composite fibers such as carbon-graghite at differentangles relative to one another and bonding them with a resin. The buttsection 20 may be formed with parallel, tapered or stepped sidewalls asdesired. The reduced diameter portion 32 is formed at one end of thebutt section 20 during lay-up or by grinding away a pre-selected annularamount of the material at one end thereof.

[0056] The hollow cylindrical tip section 18 is formed to a given lengthby drawing a blank of metallic material such as a high strength steel oraluminum through a mandrel. The tip section 18 may be formed withparallel, tapered or stepped sidewalls as desired. The length and weightof the tip section 18 is selected as described above.

[0057] An adhesive is deposited on at least one of the reduced diameterportion 32 and the inside of the tip section 18. The reduced diameterportion 32 is then telescopically inserted within the tip section 18. Asillustrated in FIGS. 9 and 10, an insulating layer 52 may be insertedbetween the tip section 18 and the butt section 20 to prevent galvaniccorrosion within the joint 30.

[0058] The foregoing relates to preferred exemplary embodiments of thepresent invention, it being understood that other embodiments andvariants thereof are possible within the scope of the invention, thelatter being defined by the appended claims.

What is claimed is:
 1. A shaft comprising: a tip section formed of a metal material and having a length between about 6 and about 12 inches and a diameter at an upper end thereof between about .0.385 and about .0.415 inches; and a butt section formed of a composite material coupled to said tip section.
 2. The shaft of claim I wherein said tip section has a lower end, a diameter of said lower end being about 0.335 inches or about 0.350 inches.
 3. The shaft of claim 1 wherein said tip section has a wall thickness between about 0.0125 and about 0.0175 inches.
 4. The shaft of claim 1 wherein said butt section has a wall thickness between about 0.025 and about 0.125 inches.
 5. The shaft of claim 1 further comprising: an adhesive disposed between said tip section and said butt section.
 6. The shaft of claim 5 wherein said adhesive has a thickness between about 0.003 and about 0.006 inches.
 7. The shaft of claim 1 wherein said butt section has a plug telescopically received within said tip section.
 8. The shaft of claim 1 wherein said tip section overlaps said plug over a length of about 0.75 to about 1.5 inches.
 9. The shaft of claim 1 further comprising a plastic ferrule disposed between said tip section and said butt section.
 10. The shaft of claim 1 further comprising: an insulating layer disposed between said tip section and said butt section.
 11. The shaft of claim 10 wherein said insulating layer further comprises one of a plurality of insulating spacers disposed between said tip section and said butt section, and a glass layer formed as an outboard part of said tip section so as to be located proximate said butt section.
 12. A shaft comprising: a tip section formed of a metal material; and a butt section formed of a composite material and having a plug extending from one end thereof telescopically received within said tip section, said tip section overlapping said plug by a length between about 0.75 and 1.5 inches.
 13. The shaft of claim 12 wherein said tip section has a length between about 6 and about 12 inches.
 14. The shaft of claim 12 wherein said butt section has a wall thickness between about 0.025 and about 0.125 inches.
 15. The shaft of claim 12 wherein said tip section has a wall thickness between about 0.0125 and about 0.175 inches.
 16. The shaft of claim 12 wherein said tip section has an upper end and a lower end, a diameter of said lower end being about 0.335 inches or about 0.350 inches.
 17. The shaft of claim 12 wherein said tip section has an upper end and a lower end, a diameter of said upper end being between about 0.385 inches and about 0.415 inches.
 18. The shaft of claim 12 further comprising: an adhesive disposed between said tip section and said butt section.
 19. The shaft of claim 18 wherein said adhesive has a thickness between about 0.003 and about 0.006 inches.
 20. The shaft of claim 12 further comprising a plastic ferrule disposed between said tip section and said butt section. 